Conventionally, a programmed temperature controller has compared the measured temperature value of a control thermometer (in general, measuring an atmospheric temperature) against a set value of a program pattern for effecting calculation of a two-position or PID or the like control, and for controlling the heat quantity from a heat source through regulating means such as thyristors, burners, and the like.
More particularly, with a programmed temperature controller having a controlled temperature input from a thermocouple or the like and a control or manipulating of an output after PID operation, it has been a practice to produce a certain program pattern a combination of a slope of a temperature change with time (hereinafter referred to as "slope") and keeping of a constant temperature for a period of time (hereinafter referred to as "keep time") and to proceed with a temperature control in accordance with the program pattern.
The temperature of the workpiece being heat treated, inclusive of an inside temperature thereof, however, does not always follow the controlled temperature, because the workpiece has a finite value of thermal capacity. Referring to FIG. 2, when the workpiece is in an atmosphere of controlled temperature TC1, there is produced in the workpiece quickly-responding portions at quick-response temperature TC2 and slowly-responding portions at a slow-response temperature TC3. The difference between the quick-response temperature TC2 and the slow-response temperature TC3 seriously affects the quality of the treated workpiece. Hence, conventional programmed temperature controllers use, apart from the controlled temperature input, a plurality of additional temperature information relating to the workpiece being heat treated. The additional temperature information is, for instance, indicated and recorded by a suitable recorder and temperature distribution in the entirety of the workpiece inclusive of the inside thereof is shown, and the slope of the program pattern is adjusted manually by an operator so as to keep the temperature distribution of the workpiece within an allowable range.
Thus, when heat treatment of a workpiece is effected through a temperature rise or descent by a conventional programmed controller (with a constant slope), it is not possible to ensure that the temperature distribution of the entirety of the workpiece inclusive of the inside thereof is always within a certain allowable range regardless of the weight, shape, disposition and way of overlaying the works one on another.
Further, with a conventional programmed temperature controller with a controlled temperature input and a control output after PID operation, temperature distribution in the workpiece being heat treated is not considered. Hence, monitoring of a temperature distribution in the workpiece is manually effected by a human operator through the use of a recorder or the like. Without a human operator, it has been impossible to ensure that a temperature distribution is always within an allowable range. When such control of the work temperature distribution is necessary, it has been inevitable that the conventional programmed temperature controller becomes a mere semi-automatic controller.